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FIELD TRIP SIDETRIP O - Lake Ann Trail; Mount Baker Highway (State Route 542)

Lake Ann Trail

Chert block
Highly deformed, metamorphosed banded chert (of the Bell Pass MÚlange) in a block along the trail to Lake Ann. Pocket knife for scale.

A geologic panarama of the last 225 million years

Perhaps one of the most rewarding scenic and geologic hikes in the Mount Baker area is the Lake Ann Trail. At the divide between Swift Creek and the cirque of Lake Ann (about 3.8 miles from the trailhead), pause to take in the geologic scene. A slow pirouette will bring into view geologic events reaching back 225 million years. An even better view is afforded by climbing a short ways on a primitive trail up the ridge to the south. From either viewpoint, start with the view west, back down the trail. Shuksan Arm, the ridge north of the trail, on the right, is held up by the oldest rocks in view, sedimentary rocks of the Chilliwack River terrane. Closer to the viewer on the same ridge, but up the slope to the northwest, a slab of chert was thrust over the sedimentary rocks along a fault. This chert is part of the Bell Pass mélange. Blocks of the mangled recrystallized chert are scattered along the trail to Lake Ann in the basin to the west of this point. Look for white blocks made up of broken lenses and layers of quartz. Some blocks look like weathered, coarsely-grained wood.

Mount Shuksan
Mount Shuksan rises above the Curtis Glacier. Shuksan Greenschist overlying Darrington Phyllite and Bell Pass Melange along thrust faults. See diagram below.

These are metamorphosed finely bedded chert and shale (banded cherts) of the ocean bottom. On Shuksan Arm,the mangled chert and shale are bound on the east by the Shuksan Thrust Fault which cuts across the ridge almost directly north of this view point.Above and east of the thrust is a slab of phyllite, overlain, going eastward, by the Shuksan Greenschist, which makes up the main bulk of Mount Shuksan. The phyllite and greenschist are in the Easton terrane, and geologists think these rocks began in the ocean as shale and basalt about 150 million years ago (Jurassic). The same sequence of tilted layers also occurs on ridges to the south. The viewer stands here in a bite out of the edge of a stacked sandwich of thrust sheets which are tilted down to the east. A lot of geologic time is not represented by the views here because the next youngest rock that can be seen is the granitic rock of the Lake Ann stock, a small pluton that intruded the older rocks about 2.2 million years ago (Pliocene). The granitic rock, part of the Cascade Volcanic Arc, is underfoot as well as exposed on nearby slopes. A lot of things happened between 150 and 2.2 million years ago, including descent of the Easton terrane to great depths in the Earth, uplift of the terrane, travel to somewhere in western North America, assemblage with other terranes by thrust faulting, and eventual arrival of the stacked terranes at a place on Earth that we now call western Washington. To the west, across the headwaters of Swift Creek, are white cliffs carved from ash beds of the Kulshan Caldera. The tuffs of these cliffs are 1.1 million years old, even younger than the Lake Ann stock.The black cliffs of Table Mountain represent the next youngest event, eruption of thick lavas about 300,000 years ago.

Stacked thrust plates on Shuksan
View of Mount Shuksan from west showing stacked sandwich of thrust plates tilted down to the southeast. About 2 million years ago the magma of the Lake Ann stock welled up into the stacked thrust plates. The Shuksan Greenschist and Darrington Phyllite make up the Easton terrane.
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Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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